To obtain high temperature thermal energy, by collecting solar energy, there are a parabolic trough system, a lens system and a central receiver system using a plural number of heliostats. The parabolic trough system is taken for example of the conventional art hereinafter. A common structure of the parabolic trough system is shown in FIG. 1, where 1 indicates a parabolic trough mirror, 3 indicates an absorber tube arranged at the focal line of the parabolic trough mirror 1.
In the absorber tube 3, heat transfer fluid 5, for example, water, liquid natrium, or carbon dioxide gas is introduced and said parabolic trough solar collector is oriented toward the sun. Solar energy 6 is reflected by the parabolic trough mirror 1 and collected at the absorber tube 3 arranged at the focal line of said mirror 1, and the heat transfer fluid 5 is heated to high temperature. Even though a solar collector requires no fuel, it is naturally important to manufacture compact solar collectors at a low cost. For this purpose, it is necessary to develop solar collectors with high collector efficiency. In order to improve collector efficiency, two means are taken; one means is to transmit solar energy to the absorber tube 3 as much as possible by improving reflectivity of the parabolic trough mirror 1, the other means is to decrease energy loss from the absorber tube 3 by covering the absorber tube 3 with glass tube 2 as shown in FIG. 1, and evacuating the space 4 between the absorber tube 3 and the glass tube 2 to decrease the convection heat loss discharged to the outer air from the absorber tube 3. Moreover, in order to increase solar energy absorption and, at the same time, to decrease emission from the absorber tube 3, a special film (which is generally called selective transparent film) with high visible radiation transmittance and high infrared radiation reflectivity is applied to the glass tube 2, or the surface of the absorber tube 3 is coated with special film (which is generally called a selective surface) with high visible radiation absorptance and low infrared radiation omittance.
A structure of a solar collector system has been described using an example, the parabolic trough system described above. A detailed description can be omitted for the lens system with use of convex lens or Fresnel lens and the central receiver system with a plural number of heliostats to collect solar energy by an absorber tube or by a group of absorber tubes arranged at the focal line, because they are all basically of the same structure as that of said parabolic trough system.
Solar energy obtainable on the ground changes abruptly depending on the weather and high temperature concentrating type solar collectors are subjected to abrupt thermal change (temperature change). That is, when the sun is covered with clouds temporarily, the high heat flux density of the concentrating type solar collector rapidly drops down to zero, and, when the sun appears again, the high heat flux density is instantly recovered. It is very hard to control the heat transfer fluid flow rate according to the flux of solar radiation. Therefore, in such a case, the temperature of the absorber tube 3 repeatedly falls down to the supply temperature of heat transfer fluid and rises extremely high, and an abrupt thermal shock sometimes damages the absorber tube 3. In addition, a burn-out phenomenon of heat transfer fluid 5 breaks the absorber tube 3. For the purpose of removing these defects of the concentrating type solar collector, a method shown in FIG. 2 is conventionally designed (Author: Raymond K. Burns, Report No. NASA TN D-6266: Preliminary Thermal Performance Analysis of the Solar Brayton Heat Receiver). In FIG. 2, 21 indicates an absorber tube with constrictions 23 in some portions thereof, 22 is a cover bellows, element 13 indicates a thermal energy storage material which is a changeable phase and enclosed between the absorber tube 21 and the cover bellows 22. An adequate material for thermal energy storage material 13 having a melting point close to a desirable absorbing temperature is chosen among molten salts consisting of single component or multicomponent mixtures as listed in Table 1. The thermal energy storage material 13 is molten by absorbed solar energy, which is temporarily stored by its heat of function.
TABLE 1 ______________________________________ Molten Salt Composing Ratio Melting Point Heat of Fusion Composition (mol %) (.degree.C.) (kcal/kg) ______________________________________ NaOH 100 318 76.0 KCl-LiCl 41.5-58.5 361 57.25 NaCl-MgCl.sub.2 52-48 450 77 NaCl-CaCl.sub.2 48-52 500 44 NaCl-KCl 50-50 658 91 LiF 100 848 246 ______________________________________
In case that there is no abrupt change in solar radiation, solar energy is transferred to the cover bellows 22, molten thermal energy storage material 13 and the absorber tube 21 and the heat transfer fluid 5 is heated. The effect of this structure is witnessed in the abrupt change of solar radiation. When the intensity of solar radiation decreases rapidly, the energy quantity absorbed also drops down rapidly, but the heat of fusion of thermal energy storage material can heat the heat transfer fluid 5 and, by decreasing gradually the flow rate of the heat transfer fluid 5, abrupt thermal change is prevented. When the intensity of solar radiation is recovered, the energy quantity absorption increases rapidly, but the temperature of the absorber tube does not rise instantly because the energy quantity is absorbed as heat of fusion of the thermal energy storage material 13. The cover bellows 22 is made in bellows shape and the absorber tube 21 has constrictions 23 for the purpose of absorbing their thermal expansion and volume change of thermal energy storage material 13.
Such a conventional solar collector has the defect that the collector efficiency lowers in order to increase radiation loss and difficulty in processing selective surface raises the cost of the solar collector. The collected thermal energy which has reached the cover bellows 22 is conducted through the thermal energy storage material 13 and transferred to the heat transfer fluid 5 through the absorber tube 21. Accordingly, the heat conductive resistance of the thermal energy storage material 13 increases and its heat transfer performance is inferior to that of the structure without thermal energy storage material shown in FIG. 1. In addition, if the temperature of heat transfer fluid is set at the same temperature, surface temperature of the cover bellows 22 must be heated to considerably higher than the surface temperature of the absorber tube 3. As is clear from FIG. 2, the cover bellows 22 is in bellows shape which makes the surface dimension extremely large. It is known that the radiation heat loss is proportional to the fourth power of its absolute temperature and proportional to its dimension. Consequently, this conventional embodiment has the defects that radiant heat loss increases and the solar collector efficiency decreases.